The Heat Treatment Of Bevel Gears
Bevel gear is one important type among all the different categories of gears. It is a gear where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped. This kind of gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well.
The expression “bevel gears” is used as a general description for straight and spiral bevel gears as well as hypoid gear sets. If the axes of the pinion and gear do not intersect but have a distance in space, the gear set is called a hypoid gear set. The name is derived from the hyperbolic shape of the “pitch cones.” For simplification, the blanks are still manufactured with a conical shape.
The heat treatment has much effect on the bevel gears. But what is the heat treatment? heat Treatment is a process where solid steel or components manufactured from steel are subject to treatment by heating to obtain required properties, e.g. softening, normalizing, stress relieving, hardening. Heating for the purpose of hot-working as in the case of rolling or forging is excluded from this definition. That is to say, the heat treatment is mainly a method used to alter the physical and sometimes chemical properties of a material.
Heat treatment follows the soft cutting operation. The generally used low carbon steel has to be carburized on the surface by case hardening. The heat treatment is finished with the quenching operation that provides a surface hardness in the range of 60 to 63 Rc (Rockwell C). The pinion may be 3 Rc harder than the ring gear to equalize the wear and reduce the risk of scoring. The core material stays softer and more ductile, with hardness in the range of 30 to 40 Rc.
Heat treatment facility (for example: salt bath, furnace or continuous furnace) and the differences between the charges of blank material, have a significant influence on the gear distortion. The distortions from heat treatment are critical to the final hard finishing operation. In addition to the blank body distortions, heat treating also causes a distortion of the individual teeth. The spiral angle changes, the flank length curvature is reduced and the pressure angle changes. To achieve the best results, you must pay attention to the processing and handling of the parts through the furnace. Quenching the ring gears with a quench press assures good flatness of the heat-treated ring gears. For instance, the hard finishing of bevel gears should provide a good surface finish and remove flank distortions.